Depends on how much you have to spend and what sort of things he likes to do and whether as a family you put more store in thecost or the thought of the present or have gone beyond presents and just take themicky out of each other with novelty gifts.
500 bucks would get him a pretty nice new (or used) guitar unless of course he has a decent one already. I'd recommend a Mexican Telecaster if he plays electric.
That's a lot of money to spend on one birthday present. Have you asked him if he wants anything? If he drinks, I would guess that a bottle of wine, whisky, whatever would be welcome.